In applications with line interface circuits, LICs, for interfacing to subscriber stations or subscriber telephone sets with a telephone network it is necessary to be able to send a ringing signal to the subscriber which is connected to a specific LIC. The ringing signal is sent as a transversal signal between the two wires, A and B, which constitute the interface towards the subscriber's station. The lowest allowable amplitude of the ringing signal is dependant on the length of the subscriber's line, and on the impedance of the bell or tone ringer in the subscriber's station at the ringing frequency. The requirement is that the ringing signal must have a root-mean-square value of at least 40 V above the bell. The bell constitutes a pure AC load on the line which has a very high resistance for DC when the subscriber set is on-hook.
The frequency of the ringing signal is specific to different countries and is as a rule between 20 and 50 Hz. In present applications, the ringing signal is substantially purely sinusoidal comprising substantially no higher-order harmonics in order to avoid interference with adjacent lines.
Two major types of ringing can be distinguished, balanced ringing and unbalanced ringing.
In unbalanced ringing, one of the wires is maintained at a stable or substantially constant potential, whereas on the other wire, the ringing signal is superimposed on a DC level. In some cases during ringing, the A-wire is kept at the stable potential and in other cases the B-wire. Unbalanced ringing is the most common type of ringing and is used e.g. in the U.S. and in many Asian countries.
In balanced ringing, the ringing signal is divided between the two wires. Normally, half the signal is applied onto the A-wire and half onto the B-wire, the common mode voltage being maintained at a constant level. Balanced ringing is relatively unusual but is used e.g. in France and in certain African countries.
For both types of ringing, the DC levels of the two wires have to be separated by an offset voltage, called ring trip battery, in order to be able to detect a DC load on the line, i.e. when someone lifts the handset during ringing.
It is specified for all countries that one of the wires is to be maintained at a given potential from a DC point of view (e.g. in the US, the A-wire is to be maintained at about ground potential) whereas the other wire shall have a DC level which is one ring trip battery from the DC level of the A-wire. This requirement stems from the fact that it should be ensured that the line current (DC) will be high enough when someone lifts the handset during ringing.
The ring trip battery used in systems of today corresponds to the DC battery at open line when the circuit is in idle mode (normally around 50 V but depending on the country). Since the ringing signal is sinusoidal (a form factor of 1.41) one of the wires will exceed ground potential sometime during the ringing period.
As an example, in the U.S., the A-wire remains at ground potential while the B-wire rings. At an infinitesimal line (0.OMEGA.), the peak value of the ringing signal will be 56.5 V in order to obtain 40 V r.m.s. across the bell. For this solution, the ring trip battery is normally around 48 V. Thus, the B-wire will exceed ground potential during some part of the ringing period for this solution. In order to manage normal line lengths, the signal level of the ringing signal has to be further increased (the line adds 180 .OMEGA./km).
In telephone systems of today, the ringing signal is applied to the line by connecting an external ringing generator by means of relays. This generator is very expensive and is often shared by a plurality of lines in order to reduce the cost per line. This results in that the reliability of the ringing function will be poor owing to the fact that a defective ringing generator will render many lines inoperative. Also, the ringing relays are relatively expensive and occupy a considerable area on the line interface board on which the LIC is fitted. This results in that fewer lines can be located on such a board, implying high overhead costs per line. Moreover, an external, ring-trip resistor network is needed to detect when someone lifts the handset during ringing.
A trend today in telephone systems is to move towards smaller modules (remote subscriber stages). This results in that small magazines containing LICs for few lines are obtained. Of course, it is possible also in these systems to have the ringing generator shared by the lines of the magazine but to the price of a very high overhead cost.
Another trend is to have large magazines containing many lines. For magazines of this type, the packing density of the line interface board will be vital. Bulky ringing connection relays and ring trip networks occupy a lot of space on the board surface. In this scenario, the ringing generator can be shared by many lines and will not be so costly, however to the price of a poor reliability of the ringing function.
Efforts have been made to produce LICs by means of which the ringing signal can be applied directly onto the line. One attempt has been to place a relay substitute module on silicon, via which an external ringing generator could be connected. There are, however, several problems associated with this solution. The relays have to be of very low resistance when the LIC is connected, which is difficult to implement. Moreover, the ringing relays must be able to withstand high voltages. Furthermore it should be pointed out that a ringing generator as well as external ring trip resistor networks still are needed which is one of the major problems.
A method and arrangement for generating a ringing signal is disclosed in U.S. patent U.S. Pat. No. 5,553,132 corresponding to the published International patent application WO-A1 95/20290, this patent application being incorporated herein by reference. The currents provided to the subscriber lines are directly modulated for providing the ringing signal. The current input terminal of a subscriber's line interface receives the output signal of an operational amplifier and the output signal of a low level signal generator, resulting in providing to the current input terminal the sum of a first current generated by a voltage which is a function of an output voltage of the line interface circuit and which is applied over a first resistor, and a second current generated by the output of the signal generator applied over a second resistor.
Another method for generating ringing signals is disclosed in British patent application GB-A 2 113 044, where also a ringing current is directly injected into the lines as derived from a low level signal generator.